Mandrill doesn't offer that feature. You can read more here
From their documentation:
The MailChimp |FORWARD| merge tag generates a link to an online form where recipients can share your email with others. Since Mandrill emails are generated and tracked individually, a web version isn't provided automatically. To mimic the Forward to a Friend option in Mandrill, you could create your own online version of the email, host it on your website, and then give your recipients a way to share that online version with friends (through a direct link or an online form). Then you could track when people fill out the form or when they click the link for the shared email.